10 CFR 50 Appendix B, Criterion III, Design Control, states, in part, that measures shall be established to assure that applicable regulatory requirements and the design basis are correctly translated into specifications, drawings, procedures, and instructions. Contrary to this, from initial construction until November 16, 2012, Dominion failed to ensure that Unit 2 safety related equipment would perform their safety function during a 22 foot
MSL design basis flood event concurrent with a 26.5 foot
MSL standing wave in the intake structure. Specifically, the unsealed electrical conduits and other openings would have allowed water to bypass Dominions flood protection features and could have affected the functionality of the safety related
AFW and
HPSI pumps and the
PORVs. Dominion entered the issue into their corrective action process as CR491792 and sealed the conduits. Dominion performed an analysis that modeled the postulated effects of the compromised flood barriers. The evaluation postulated the time based impact of the design basis Probable Maximum Hurricane (
PMH) tidal surge, using data (including wave runup above the still water heights) from Table 2.5-1 of the
UFSAR, with and without the concurrent +26.5 ft
MSL water level in the intake structure. The calculation estimated the height of water in the turbine, control, and auxiliary buildings rooms containing equipment necessary to maintain safe hot shutdown using: physical plant layout (floor areas and elevations, internal access doors and postulated water flow paths); water flow estimates; relative height of the identified leakage points; and critical water levels where equipment could be compromised. The engineering calculations demonstrated no impact to equipment needed to perform during the design basis flood without the concurrent intake structure standing wave. However, there was a potential to affect the functionality of the
auxiliary feedwater pumps, the
PORVs and the high pressure injection system if the standing wave condition occurred, as assumed, for one hour concurrent with the design basis maximum storm surge. The inspectors and a Region I senior risk analyst (
SRA) reviewed the associated engineering calculations and technical evaluation. The Region I
SRAs conducted and peer reviewed a detailed risk evaluation which they discussed with Office if Nuclear Reactor Regulation, Division of Risk Assessment staff. The
SRAs determined that the finding was of very low safety significance with an estimated increase in core damage frequency of less than one in one million reactor years (Green). This was based on available frequency information and on the possibility of some credit for core damage mitigation equipment due to conservative assumptions, as follows: Dominion included significant conservatisms in their calculation and evaluation, which tend to overestimate the chance of damage to mitigation equipment, such as: including wave runup above the assumed still water heights; the one hour duration of intake structure water level at + 26.5 ft
MSL due to the postulated standing wave; the height at which equipment damage would occur; and the assumed size of the identified flood barrier breaches. Dominion took no credit for operator actions to protect the important equipment either prior to or during a predicted extreme weather event. Plant procedures for these types of weather conditions discuss pre-staging equipment (sand bags, portable pumps and generators) and personnel to respond to limit the impact of potential flooding on important equipment.
